i5 3570k can be overclocked
get z77 series motherboard, you will be able to overclock that way
1155 Asus P8B75-M LX (USB3 - 2 x DDR3 1333 - Gig Lan - SATA 2 - USB2 & 3)
1155 Gigabyte GA-Z77-D3H (Z77 chipset - 4 x DDR3 2200 - Gig Lan - SATA 2 & 3 - USB 3 & 2)
The system comes to £584.54
Is that a decent enough system to run games well?